Once-through, forced-flow boilers



J1me 1966 P. H. PACAULT ETAL 3,255,735

ONCETHROUGH FORCED-FLOW BOILERS Filed Dec. 27. 1965 5 Sheets-Sheet 1 FIG. 1

INV EN TOR.

Pierre H. Pacaulf 3y Gaston J. Surrel ATTORNEY June 14, 1966 P. H. PACAULT ETAL 3,255,735

ONCE-THROUGH, FORCED-FLOW BOILERS Filed Dec. 27. 1963 5 Sheets-Sheet 2 FIG.2

Dq VJV June 1966 P. H. PACAULT ETAL 3,255,735

ONCE-THROUGH, FORCED-FLOW BOILERS 5 Sheets-Sheet 3 Filed Dec. 27. 1963 June 1966 P. H. PACAULT ETAL 3,255,735

ONCE-THROUGH, FORCED-FLOW BOILERS Filed Dec. 27, 1965 5 Sheets-Sheet 4 J1me 1966 P. H. PACAULT ETAL 3,255,735

ONCE-THROUGH FORCED-FLOW BOILERS Filed Dec. 27, 1963 5 Sheets-Sheet 5 I 0: cu N 0 I: q

S- wi L United States Patent @NCETHRQUGH, IFGRCED-FLOW BUiiiEltS Pierre Henri Pacault, Paris, and Gaston .Iules Surrel,

Molsheirn, France, assignors to Eabcoclr dz 'Wilcox Limited, London, England, a corporation of Great Britain Filed Dec. 27, 1963, Ser. No. 335 ,9156 6 tliaims. (Cl. 122-4ti6) The present invention relates to once-through, forcedflow boilers.

In starting up such a boiler, or in running it at low,

loads, there is a danger that the fluid heating circuitry will become overheated since the fluid flow therethrough, when the steam consumption is nil or small, may be too low to remove adequately the heat supplied to the circuitry by the burners.

In one method of starting up a once-through, forcedflow boiler, fluid that has been heated by passing through the fluid heating circuitry is expanded to provide a vapor phase, that may be used in a low pressure turbine, and a liquid phase that, being cooled by the expansion, is recirculated to the fluid heating circuitry. Objections to this arise from the large size of the equipment required for the expansion of the heated fluid and the separation of the phases and from the fact that the temperature at which liquid is returned to the fluid heating circuitry may be considerably higher than that at which fresh fluid is supplied to the same circuitry. It may be, too, that for low load operation, the boiler itself may not be suitable for the generation of low pressure vapor which would have to be provided by the recirculation which, generally, can provide vapor in large quantities only inefliciently.

According to the present invention, there is provided a vapor generating plant including a once-through forcedflow boiler, a surface-type heat exchanger, means for passing fluid that hasbeen heated in the boiler through the primary side of the heat exchanger as heating fluid, means for passing a coolant fluid through the secondary side of the heat exchanger in indirect heat transfer relation with the fluid flowing through the primary side of the heat exchanger, and means for recirculating to the boiler fluid that has flowed as heating fluid through the heat exchanger.

The present invention also provides a system for starting up, or operating at low load, a once-through, forcedflow boiler in which fluid that has been heated in the boiler is cooled in a surface-type heat exchanger and fluid that has been cooled in the heat exchanger is recirculated to the boiler.

The heat extracted in the heat exchanger from the heating fluid may be used for the generation of low pressure vapor, and this may be used to facilitate the starting up of a turbine, or for preheating combustion gases used in firing the boiler. The heat extracted in the heat exchanger may also be such as to ensure that the temperature of the pump by which fluid is returned from the heat exchanger to the heating surfaces of the boiler remains low.

By way of example, embodiments of the invention will now be described with references to the accompanying schematic drawings in which:

FIGURE 1 shows a plant including a once-through, forced-flow boiler and a surface type heat exchanger arranged for the production of low-pressure vapor.

FIGURE 2 indicates a modification of the plant shown in FIGURE 1;

FIGURE 3 indicates a further modification of the plant shown in FIGURE 1;

FIGURE 4 indicates a further modification of the plant shown in FIGURE 1; and

Patented June 14, '1956 FIGURE 5 indicates a modification of the plant shown in FIGURE 2.

In FIGURE 1, the symbols P, N, T and Q respectively designate control devices that are responsive to the pressure, level, temperature and output.

In the plant shown in FIGURE 1, power is generated in a turbine providing a high pressure stage HP, a medium pressure stage MP and a low pressure stage LP. Vapor from the low pressure stage LP passes to a condenser OR, the condensate being circulated by an extraction pump PE through a low pressure heater RL to a deaerator and storage drum SD. A unit PC for conditioning the operating fluid is connected in parallel with the pump PE and the heater RL. 'From the storage drum SD, operating fluid is pumped by a feed pump PF through a conduit 35 and a high pressure feed heater R-H to the once-th-rough, forced-flow boiler.

The boiler includes economizer surfaces 1, vapor generating surfaces 2, primary and secondary superheating surfaces 3 and 4, respectively, and a reheater 5. Fluid is fed to the economizer surface 1 by an electrically driven feed pump 22. A by-pass 6, controlled by a valve 7 and containing a back pressure resistance 8, is connected between the superheaters 3 and 4; and a further by-pass 9, controlled by a valve 10, is connected between the secondary superheater and the high pressure stage I-IP of the turbine. Both by-passes 6 and 9 lead to a single conduit 11 from which fluid flows through the primary side, comprising heating fluid tubes 12a, of a surface type heat exchanger 12. A pump 14 is provided to return fluid from the heat exchanger 12 through a conduit 13 to the upstream end of the vapor generating surf-aces 2. A bypass, controlled by a valve 17, enables fluid flowing through the conduit 11 to be expanded into the coolant space or secondary side of the heat exchanger 12.

Water is led as coolant from conduit 30 through a conduit 15 into the coolant space or secondary side of the heat exchanger 12 to be vaporized by indirect heat exchange with the fluid flowing through the tubes 12a. The outlet from the coolant space is provided by a conduit 16 to which are connected four valve-controlled flow paths 18, 19, 20 and 21 which enable vapor produced in the coolant space of the heat exchanger 12 to be used in various ways.

Vapor passing through the flow path 18 serves to seal the glands of the turbine and may also be used to drive turbine until the load may be placed on the boiler. Vapor flowing through the flow path 18 also serves to protect the reheater 5 until vapor flows through it at a sufficient rate from the high pressure stage HP of the turbine.

The flow path 19 leads vapor from the conduit 16 to the condenser CR and the flow of vapor through the flow path can be at any rate that is necessary to prevent the temperature of the heating surfaces '2 from becoming too high. Flow path 20 leads to the deaera-tor and storage drum SD, while flow path 21 leads vapor as heating fluid over the high pressure heater RH from which the condensate flows back to the storage drum SD.

Suppose the plant shown in FIGURE 1 is to be started up from cold. Pump 22 is operated to send operating fluid through the vapor generating circuitry 2. Valves 7 and 10 are fully open so that all the fluid flowing through vapor generating circuitry 2 will enter the by-passes 6 and 9. To ensure circulation in the boiler, for blowdown, and to enable the recirculated fluid to pass through the conditioning plant FC, the valve 17 is initially fully open. It is then partially closed and the burners firing the boiler are started at a low rate. At the same time, pump 14 is operated to recirculate to the surfaces 2 the operating fluid that does not pass through the valve 17, the rate at which fluid is recirculated by the pump 14 being such as to prevent the surfaces 2 from becoming overheated.

As soon as the temperature of the fluid leaving the surfaces 2 exceeds 100 C., the water conditioning unit FC is cut out so that hot fluid can no longer flow through it and cause it to deteriorate.

Part of the fluid flowing through the valve 17 vaporizes and the vapor is sent through the flow path 18 to seal the glands of the turbine while the remainder passes to the condenser CR and the storage drum SD, the proportions in which the vapor is divided amongst the flow paths 18,

19, 20 and 21 being such as to ensure the best values for the pressure, temperature and chemical conditions, such as oxygen content, of the feed to the boiler.

As the valve 17 is gradually closed, the flow of coolant through the conduit 15 is correspondingly increased. The coolant will be vaporized by indirect heat exchange with the fluid flowing through tubes 12a, the vapor derived from the coolant being added to the vapor resulting from expansion at the valve 17. Eventually, the valve 17 will be completely closed, and increases in the rate of firing of the boiler will be accompanied by increases in the rate at which fluid flows to the heat exchanger 12 through the conduit 15. The vapor generated in heat exchanger 12 and supplied to the medium pressure stage of the turbine through the flow path 18 increases the speed of the turbine until synchronous speed is reached when part of the load is taken.

During the starting-up process, the rate at which fluid flows through the by-passes is regulated until the turbine is able to operate normally with the vapor that the high pressure stage HP receivess from the superheater 4 at the appropriate pressure and temperature. During normal operation, the path 18 may be closed and any excess vapor produced in the heat exchanger is led to the condenser CR. In regulating the fiow through the by-passes 6 and 9, care must be taken to ensure that there is a sufficient flow through the tubes 12a to enable the surfaces 2 to be adequately cooled and to ensure that the pumping temperature at 14 remains at a suitable value.

In operation of the plant shown in FIGURE 1 at low loads, the rate at which-fluid must be supplied to the boiler may be inadequate to ensure that the heating tubes do not become overheated so that recirculation of fluid through the pump 14 may be resorted to during the low load operation.

In some methods of operating the plant shown in FIG- URE 1, low pressure vapor may continue to be supplied to the turbine from the heat exchanger 12 even though vapor from the secondary superheater is being .supplied to the high pressure stage of the turbine. The total heat absorbed by the boiler is thus divided between the provision of high pressure vapor and low pressure vapor. The controls effected in the method described have been concerned mainly to provide a desired high pressure vapor output and low pressure vapor output during' starting up. The total output of the boiler may be varied by varying, for instance, the fuel consumption, the high pressure vapor output relatively to the low pressure vapor output, and the flows through the by-passes. Other methods of using the plant desscribed may be such as to control the position of the transition zone or steam temperatures. The methods may also be adapted to operate within prescribed lim- 'its of combustion power, total output of fluid from the heating surfaces that are to be protected, the temperature of fluid at the outlet from the combustion chamber and the recirculation pump temperature.

In the modified plant indicated in FIGURE 2, all the vapor used in the turbine is produced at the vapor generating surfaces 2 and the heat exchanger 24 serves to heat air that is supplied as combustion air to the boiler burners.

FIGURE 3 indicates a modification of the plant shown in FIGURE 1 in which a further heat exchanger 25 is incorporated. In the heat exchanger 25, heat is extracted from the fluid leaving the tubes 12a by feed fluid p'asssing to the economizer 1. This enables an even lower pumping temperature to be obtained than with the plant shown in FIGURE 1 without sacrificing the other flexibility of adjustment that is obtainable with that plan.

In the modification indicated in FIGURE 4, an auxiliary heat exchanger 26 is placed upstream of the heat exchanger 12 and the heat is extracted from the fluid flowing to the heat exchanger 12 by fluid returning from that heat exchanger to the vapor generating surfaces 2. Since the fluid flowing to the heat exchanger 12 is partially cooled before reaching the heat exchanger 12, the heat exchanger 12 can cool the fluid to a lower temperature than would otherwise be possible so that, again, a low pumping temperature can .be achieved.

The plant indicated in FIGURE 5 is comparable with that shown in FIGURE 2 except that the heat exchanger 24, which serves to heat combustion air, is replaced by a heat exchanger 27 which serves to heat fluid flowing to the economizer 1.

While in accordance with the provisions of the statutes we have illustrated and described herein the best form and mode of operation of the invention now known to us, those skilled in the art will understand that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention covered by our claims, and that certain features of our invention may sometimes be used to advantage without a corresponding use of other features.

What is claimed is:

1. In a power plant having a turbine, a forced flow boiler having a through-flow circuit including a fluid heating section and a superheater section connected for series flow from the fluid heating section and to the turbine, means for supplying vaporizable fluid under substantial pressure to the fluid heating section, means for starting-up the boiler and turbine comprising a heat exchanger for generating vapor having a primary side and a secondary side, means for by-passing a portion of the circuit flow from a position intermediate the turbine and the fluid heating section to and through the primary side of the heat exchanger, means for by-passing a portion of the fluid inflow to said fluid heating section to the secondary side of the heat exchanger at a pressure considerably less than the fluid heating section inflow pressure and in indirect heat absorbing relation with the fluid passing through the primary side of the heat exchanger to generate low pressure vapor in the secondary side of the heat exchanger, means for recirculating the fluid outflow .of the primary side of the heat exchanger to the fluid heating section, and means for passing low pressure vapor from the secondary side of the heat exchanger to the turbine.

2. In a power plant having a turbine, a forced flow boiler having a through-flow circuit including a vapor generating section and a superheater section connected for series flow from the vapor generating section and to the turbine, means for supplying vaporizable fluid to the vapor generating section, means for starting-up the boiler and turbine comprising a heat exchangerfor generating vapor having a primary side and a secondary side, means for passing a portion of the superheater section outflow from a position intermediate the turbine and the superheater section to and through the primary side of the heat exchanger, means for by-passing a portion of the fluid inflow to said vapor generating section to the secondary side of the heat exchanger in indirect heat absorbing relation with the fluid passing through the primary side of the heat exchanger to generate vapor in the secondary side of the heat exchanger, means for recirculating the fluid outflow of the primary side of the heat exchanger to the vapor generating section, and means for passing vapor from the secondary side of the heat exchanger to the turbine.

3. In a power plant as claimed in claim 2, in which the turbine includes two stages, a reheater is connected 5 between the stages and means are provided for passing vapor from the secondary side of the heat exchanger to and through the reheater.

4. In a power plant as claimed in claim 1, in which means are provided for discharging fluid that has been heated in the fluid heating section of the boiler into the secondary side of the heat exchanger.

5. In a power plant as claimed in claim 1, in which the means for recirculating fluid to the fluid heating section of the boiler includes a pump located between the heat exchanger and the fluid heating section and there is provided a second heat exchanger, and means for leading fluid that is being recirculated to the fluid heating section through the second heat exchanger in indirect heat exchange relationship with feed fluid flowing to the fluid heating section.

6. In a power plant as claimed in claim 1, in which the means for recirculating fluid to the fluid heating section of boiler includes a pump located between the heat exchanger and the fluid heating section and there is provided a second heat exchanger whereby fluid that has been heated in the fluid heating section flows through said second heat exchanger as heating fluid before passing to said heat exchanger, and means whereby fluid that is being recirculated to the fluid heating section is led through the pump and then as cooling fluid through said second heat exchanger.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCES Mittielungen Number 1, September 1956, published by Durrwerke-Aktiengesellschaft, Ratingen, Germany, pp.

KENNETH W. SPRAGUE, Primary Examiner.

FREDERICK L. MATTESON, IR., Examiner.

25 D. G. BLACKHURST, C. R. REMKE,

Assistant Examiners. 

1. IN A POWER PLANT HAVING A TURBINE, A FORCED FLOW BOILER HAVING A THROUGH-FLOW CIRCUIT INCLUDING A FLUID HEATING SECTION AND A SUPERHEATER SECTION CONNECTED FOR SERIES FLOW FROM THE FLUID HEATING SECTION AND TO THE TURBINE, MEANS FOR SUPPLYING VAPORIZABLE FLUID UNDER SUBSTANTIAL PRESSURE TO THE FLUID HEATING SECTION, MEANS FOR STARTING-UP THE BOILER AND TURBINE COMPRISING A HEAT EXCHANGER FOR GENERATING VAPOR HAVING A PRIMARY SIDE AND A SECONDARY SIDE, MEANS FOR BY-PASSING A PORTION OF THE CIRCUIT FLOW FROM A POSITION INTERMEDIATE THE TURBINE AND THE FLUID HEATING SECTION TO AND THROUGH THE PRIMARY SIDE OF THE HEAT EXCHANGER, MEANS FOR BY-PASSING A PORTION OF THE FLUID INFLOW TO SAID FLUID HEATING SECTION TO THE SECONDARY SIDE OF THE HEAT EXCHANGER AT A PRESSURE CONSIDERABLY LESS THAN THE FLUID HEATING SECTION INFLOW PRESSURE AND IN INDIRECT HEAT ABSORBING RELATION WITH THE FLUID PASSING THROUGH THE PRIMARY SIDE OF THE HEAT EXCHANGER TO GENERATTE LOW PRESSURE VAPOR IN THE SECODDARY SIDE OF THE HEAT EXCHANGER, MEANS FOR RECIRCULATING THE FLUID OUTFLOW OF THE PRIMARY SIDE OF THE HEAT EXCHANGER TO THE FLUID HEATING SECTION, AND MEANS FOR PASSING LOW PRESSURE VAPOR FROM THE SECONDARY SIDE OF THE HEAT EXCHANGER TO THE TURBINE. 